feat(discover): wire A4 search + A5 downloads to backend

Adds DownloadJob/ExternalSearchResult/SourceInfo contract types + mappers, the downloads + search RTKQ endpoints, and the SearchDownloadPage (search external sources, per-result download states) and DownloadsManagerPage (active/history, progress, retry/cancel, poll-while-active). en/ru i18n. Snapshot also bundles in-progress queue/metadata-editor/storage UI work.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/store/rtkqPersist.ts

@@ -32,7 +32,11 @@ function snapshot(apiState: ApiState): RehydrateApiPayload {
   }
   // Carry `provided` along so RTKQ can re-register invalidation tags for the
   // restored entries; it is also required structurally (see RehydrateApiPayload).
-  return { queries, mutations: {}, provided: apiState.provided ?? EMPTY_PROVIDED };
+  return {
+    queries,
+    mutations: {},
+    provided: apiState.provided ?? EMPTY_PROVIDED,
+  };
 }
 
 function load(): RehydrateApiPayload | null {